Background

Stage 1: execution of the stage1 image from within the composite image prepared by stage0.

Stage 2: execution of individual application images within the containment afforded by stage1.

This separation of concerns is reflected in the file-system and layout of the composite image prepared by stage0:

Stage 0: rkt executable, and the pod manifest created at /var/lib/rkt/pods/prepare/$uuid/pod.

Stage 1: stage1.aci, made available at /var/lib/rkt/pods/run/$uuid/stage1 by rkt run.

Stage 2: $app.aci, made available at /var/lib/rkt/pods/run/$uuid/stage1/rootfs/opt/stage2/$appname by rkt run, where $appname is the name of the app in the pod manifest.

The stage1 implementation is what creates the execution environment for the contained applications.
This occurs via entrypoints from stage0 on behalf of rkt run and rkt enter.
These entrypoints are executable programs located via annotations from within the stage1 ACI manifest, and executed from within the stage1 of a given pod at /var/lib/rkt/pods/$state/$uuid/stage1/rootfs.

Stage2 is the deployed application image.
Stage1 is the vehicle for getting there from stage0.
For any given pod instance, stage1 may be replaced by a completely different implementation.
This allows users to employ different containment strategies on the same host running the same interchangeable ACIs.

Entrypoints

rkt run

coreos.com/rkt/stage1/run

rkt prepares the pod's stage1 and stage2 images and pod manifest under /var/lib/rkt/pods/prepare/$uuid, acquiring an exclusive advisory lock on the directory.
Upon a successful preparation, the directory will be renamed to /var/lib/rkt/pods/run/$uuid.

chdirs to /var/lib/rkt/pods/run/$uuid.

resolves the coreos.com/rkt/stage1/run entrypoint via annotations found within /var/lib/rkt/pods/run/$uuid/stage1/manifest.

It is the responsibility of this entrypoint to consume the pod manifest and execute the constituent apps in the appropriate environments as specified by the pod manifest.

The environment variable RKT_LOCK_FD contains the file descriptor number of the open directory handle for /var/lib/rkt/pods/run/$uuid.
It is necessary that stage1 leave this file descriptor open and in its locked state for the duration of the rkt run.

In the bundled rkt stage1 which includes systemd-nspawn and systemd, the entrypoint is a static Go program found at /init within the stage1 ACI rootfs.
The majority of its execution entails generating a systemd-nspawn argument list and writing systemd unit files for the constituent apps before executing systemd-nspawn.
Systemd-nspawn then boots the stage1 systemd with the just-written unit files for launching the contained apps.
The /init program's primary job is translating a pod manifest to systemd-nspawn systemd.services.

An alternative stage1 could forego systemd-nspawn and systemd altogether, or retain these and introduce something like novm or qemu-kvm for greater isolation by first starting a VM.
All that is required is an executable at the place indicated by the coreos.com/rkt/stage1/run entrypoint that knows how to apply the pod manifest and prepared ACI file-systems.

The resolved entrypoint must inform rkt of its PID for the benefit of rkt enter.
Stage1 implementors have two options for doing so; only one must be implemented:

/var/lib/rkt/pods/run/$uuid/pid: the PID of the process that will be given to the "enter" entrypoint.

/var/lib/rkt/pods/run/$uuid/ppid: the PID of the parent of the process that will be given to the "enter" entrypoint. That parent process must have exactly one child process.

The entrypoint of a stage1 may also optionally inform rkt of the "pod cgroup", the name=systemd cgroup the pod's applications are expected to reside under, via the subcgroup file. If this file is written, it must be written before the pid or ppid files are written. This information is useful for any external monitoring system that wishes to reliably link a given cgroup to its associated rkt pod. The file should be written in the pod directory at /var/lib/rkt/pods/run/$uuid/subcgroup.

The file's contents should be a text string, for example of the form machine-rkt\xuuid.scope, which will match the control in the cgroup hierarchy of the ppid or pid of the pod.

Any stage1 that supports and expects machined registration to occur will likely want to write such a file.

--interactive to run a pod interactively, that is, pass standard input to the application (only for pods with one application)

--local-config=$PATH to override the local configuration directory

--private-users=$SHIFT to define a UID/GID shift when using user namespaces. SHIFT is a two-value colon-separated parameter, the first value is the host UID to assign to the container and the second one is the number of host UIDs to assign.

--mutable activates a mutable environment in stage1. If the stage1 image manifest has no app entrypoint annotations declared, this flag will be unset to retain backwards compatibility.

Arguments added in interface version 2

--hostname=$HOSTNAME configures the host name of the pod. If empty, it will be "rkt-$PODUUID".

Arguments added in interface version 3

--disable-capabilities-restriction gives all capabilities to apps (overrides retain-set and remove-set)

Arguments added in interface version 4

--dns-conf-mode=resolv=(host|stage0|none|default),hosts=(host|stage0|default): Configures how the stage1 should set up
the DNS configuration files /etc/resolv.conf and /etc/hosts. For all, host means to bind-mount the host's
version of that file. none means the stage1 should not create it. stage0 means the stage0 has created an entry
in the stage1's rootfs, which should be exposed in the apps. default means the standard behavior, which for
resolv.conf is to create /etc/rkt-resolv.conf iff a CNI plugin specifies it, and for hosts is to create
a fallback if the app does not provide it.

Arguments added in interface version 5 (experimental)

This interface version is not yet finalized, thus marked as experimental.

--mutable to run a mutable pod

Arguments added in interface version 6

--ipc=[auto|private|parent] (default to auto if missing): Allows to stay in the host IPC namespace. The default
to auto does what makes more sense for the flavor (parent for stage1-fly and private for stage1-coreos
and stage1-kvm).

rkt enter

coreos.com/rkt/stage1/enter

rkt verifies the pod and image to enter are valid and running

chdirs to /var/lib/rkt/pods/run/$uuid

resolves the coreos.com/rkt/stage1/enter entrypoint via annotations found within /var/lib/rkt/pods/run/$uuid/stage1/manifest

In the bundled rkt stage1, the entrypoint is a statically-linked C program found at /enter within the stage1 ACI rootfs.
This program enters the namespaces of the systemd-nspawn container's PID 1 before executing the /enterexec program.
enterexec then chroots into the ACI's rootfs, loading the application and its environment.

An alternative stage1 would need to do whatever is appropriate for entering the application environment created by its own coreos.com/rkt/stage1/run entrypoint.

Arguments

--pid=$PID passes the PID of the process that is PID 1 in the container.
rkt finds that PID by one of the two supported methods described in the rkt run section.

--appname=$NAME passes the app name of the specific application to enter.

the separator --

cmd to execute.

optionally, any cmd arguments.

rkt gc

coreos.com/rkt/stage1/gc

The gc entrypoint deals with garbage collecting resources allocated by stage1.
For example, it removes the network namespace of a pod.

Arguments

--debug to activate debugging

UUID of the pod

Arguments added in interface version 5

--local-config: The rkt configuration directory - defaults to /etc/rkt if not supplied.

rkt stop

coreos.com/rkt/stage1/stop

The optional stop entrypoint initiates an orderly shutdown of stage1.

In the bundled rkt stage 1, the entrypoint is sending SIGTERM signal to systemd-nspawn. For kvm flavor, it is calling systemctl halt on the container (through SSH).

Arguments

--force to force the stopping of the pod. E.g. in the bundled rkt stage 1, stop sends SIGKILL

UUID of the pod

Crossing Entrypoints

Some entrypoints need to perform actions in the context of stage1 or stage2. As such they need to cross stage boundaries (thus the name) and depend on the enter entrypoint existence. All crossing entrypoints receive additional options for entering via the following environmental flags:

RKT_STAGE1_ENTERCMD specify the command to be called to enter a stage1 or a stage2 environment

RKT_STAGE1_ENTERPID specify the PID of the stage1 to enter

RKT_STAGE1_ENTERAPP optionally specify the application name of the stage2 to enter

rkt app add

(Experimental, to be stabilized in version 5)

coreos.com/rkt/stage1/app/add

This is a crossing entrypoint.

Arguments

--app application name

--debug to activate debugging

--uuid UUID of the pod

--disable-capabilities-restriction gives all capabilities to apps (overrides retain-set and remove-set)

--private-users=$SHIFT to define a UID/GID shift when using user namespaces. SHIFT is a two-value colon-separated parameter, the first value is the host UID to assign to the container and the second one is the number of host UIDs to assign.

rkt app start

(Experimental, to be stabilized in version 5)

coreos.com/rkt/stage1/app/start

This is a crossing entrypoint.

Arguments

--app application name

--debug to activate debugging

rkt app stop

(Experimental, to be stabilized in version 5)

coreos.com/rkt/stage1/app/stop

This is a crossing entrypoint.

Arguments

--app application name

--debug to activate debugging

rkt app rm

(Experimental, to be stabilized in version 5)

coreos.com/rkt/stage1/app/rm

This is a crossing entrypoint.

Arguments

--app application name

--debug to activate debugging

rkt attach

(Experimental, to be stabilized in version 5)

coreos.com/rkt/stage1/attach

This is a crossing entrypoint.

Arguments

--action action to perform (auto-attach, custom-attach or list)

--app application name

--debug to activate debugging

--tty-in whether to attach TTY input (true or false)

--tty-out whether to attach TTY output (true or false)

--stdin whether to attach stdin (true or false)

--stdout whether to attach stdout (true or false)

--stderr whether to attach stderr (true or false)

Stage1 Metadata

Versioning

The stage1 command line interface is versioned using an annotation with the name coreos.com/rkt/stage1/interface-version.
If the annotation is not present, rkt assumes the version is 1.

Runtime Metadata

Pods and applications can be annotated at runtime to signal support for specific features.

Mutable pods (experimental v5)

Stage1 images can support mutable pod environments, where, once a pod has been started, applications can be added/started/stopped/removed while the actual pod is running. This information is persisted at runtime in the pod manifest using the coreos.com/rkt/stage1/mutable annotation.

If the annotation is not present, false is assumed.

Attachable applications (experimental v5)

Stage1 images can support attachable applications, where I/O and TTY from each applications can be dynamically redirected and attached to.
In that case, this information is persisted at runtime in each application manifest using the following annotations:

coreos.com/rkt/stage2/stdin

coreos.com/rkt/stage2/stdout

coreos.com/rkt/stage2/stderr

Filesystem Layout Assumptions

The following paths are reserved for the stage1 image, and they will be populated at runtime.
When creating a stage1 image, developers SHOULD NOT use these paths to store content in the image's filesystem.

stage2

opt/stage2

This directory path is used for extracting the ACI of every app in the pod.
Each app's rootfs will appear under this directory,
e.g. /var/lib/rkt/pods/run/$uuid/stage1/rootfs/opt/stage2/$appname/rootfs.

status

rkt/status

This directory path is used for storing the apps' exit statuses.
For example, if an app named foo exits with status = 42, stage1 should write 42
in /var/lib/rkt/pods/run/$uuid/stage1/rootfs/rkt/status/foo.
Later the exit status can be retrieved and shown by rkt status $uuid.

env

rkt/env

This directory path is used for passing environment variables to each app.
For example, environment variables for an app named foo will be stored in rkt/env/foo.

iottymux (experimental v5)

rkt/iottymux

This directory path is used for TTY and streaming attach helper.
When attach mode is enabled each application will have a rkt/iottymux/$appname/ directory, used by the I/O and TTY mux sidecar.

supervisor-status (experimental v5)

rkt/supervisor-status

This path is used by the pod supervisor to signal its readiness.
Once the supervisor in the pod has reached its ready state, it MUST write a rkt/supervisor-status -> ready symlink.
A symlink missing or pointing to a different target means that the pod supervisor is not ready.